This invention relates in general to superheat controllers. In particular, this invention relates to an improved method of maintaining the flow rate of a refrigerant, such as in a conventional heating, ventilating, air conditioning, and refrigeration (HVAC-R) system, while maintaining superheat in the HVAC-R system at a desired level.
Conventional superheat control in a typical HVAC-R system uses a pressure sensor and a temperature sensor to measure HVAC-R system fluid pressure and temperature, respectively. Superheat is then calculated for a particular refrigerant using the measured temperature and pressure, and controlled by causing the superheat to move to a target superheat value by adjusting the pressure and temperature using any of a group of known open-loop or closed-loop algorithms, such as a classic proportional-integral-derivative (PID) loop.
Superheat is a function of pressure and temperature, and is conventionally calculated using pressure-temperature (P-T) charts that map a saturation temperature at a particular pressure. The values of the saturation temperatures at particular pressures may vary with different refrigerants. These values for saturation temperature and a temperature of the refrigerant are typically measured at an outlet of an evaporator in the conventional HVAC-R system, and are typically used to calculate superheat.
In conventional superheat control, system fluid pressure has been shown to oscillate, causing an undesirable uneven fluid flow rate through an electronic expansion valve (EEV) in the typical HVAC-R system, undesirably lengthening the on-time of the typical HVAC-R system. The conventional calculation of superheat can be slow to reflect an actual superheat due to the thermal properties of the system, particularly, the relatively long time required for the temperature sensor to stabilize Pressure however, responds almost instantaneously.
Thus, it would be desirable to provide an improved method of maintaining the flow rate of a refrigerant, such as in a conventional HVAC-R system, while maintaining superheat in the HVAC-R system at a desired level.